One of the most important aspects relating to the use of radioactive materials involves the disposal of waste products and by-products of radioactive material processing and use. Some of these waste and by-products can present continuing health hazards if not properly contained.
The length of time necessary for the decay of radioactive materials is typically measured in terms of the "half-life" of the particular decay mechanism. The half-life is a term used to designate the period of time during which one half of the number of original atoms in a given sample will have decayed. Although radioactive decay is a random spontaneous process, its macroscopic properties are mathematically predictable and may be experimentally determined. Thus, the half-life values are relatively well known for most common decay process steps.
The most common radioactive atoms found in waste materials and by-products are two isotopes of uranium, uranium 235 (.sub.92 U.sup.235) and uranium 238 (.sub.92 U.sup.238), and one of plutonium namely plutonium 239 (.sub.92 U.sup.238). These three materials all have, as their primary natural radioactive decay mechanism, the emission of alpha particles. Each of these isotopes will eventually decay to a stable material. The first step in the radioactive decay of plutonium 239 is the emission of an alpha particle to produce uranium 235. Thus both plutonium 239 and uranium 235 will follow the same decay pattern. The eventual resulting stable particle obtained from the decay of uranium 238 is lead 206 (.sub.82 Pb.sup.206 ), while that resulting from the decay of uranium 235 and plutonium 239 is lead 207 (82Pb.sup.207). The plutonium 239 decay chain embodies 12 steps, the uranium 238 chain as 14 steps, and the uranium 235 has 11 steps. The decay chain mechanisms for these isotopes are shown in Appendix A.
The two principle steps in the decay of the common radioactive isotopes of uranium 235, uranium 238 and plutonium 239 are emission of alpha particles and beta particles from the nucleus. Alpha particle emission occurs when an alpha particle escapes intact from the nucleus of an atom of the unstable material. An alpha particle is comprised of two protons and two neutrons. This particle is a particularly stable configuration in terms of nuclear binding forces. The emission of an alpha particle from a radioactive atom results in the lowering of the atomic number of the atom by two and a lowering of the mass number by four. Beta particle emission results from the spontaneous decay of a neutron to a proton which remains in the nucleus and an electron which is emitted therefrom and an anti-neutrino.
The result of a beta emission from a nucleus is a unit increase in the atomic number of the atom with no change in the atomic mass. For example, one step in the decay of uranium 235 to lead involves the emission of a beta particle from thorium 231 (.sub.90 Th.sup.231) to yield protactinium 231 (.sub.91 Pa.sup.231). Typically, a given nucleus will decay by either alpha emission, or by beta emission, although some nuclei may decay by other methods, including gamma emission and spontaneous fission. The half-life of beta decay is ordinarily significantly shorter than that for typical alpha decay (see Appendix A).
In the case of the three primary isotopes found in radioactive waste material and by-products, the primary limiting step in the decay is the initial alpha particle emission from the material. The half-lives for these initial decays are extremely long. The initial alpha emission for plutonium 239 has a measured half-life of 24,360 years. Uranium 235 has a half-life of 713 million years, while uranium 238 is the most stable of all, having a half-life or 4.5 billion years. The radioactive content of the waste and by-products of these materials thus remains high over a long period of time.
It is highly desirable to eliminate the radioactivity of waste materials by decontaminating such materials as quickly as possible. Although most alpha decay steps and beta decay steps present no direct hazard, some of these released particles have sufficient energy to cause harm to living things such as animals, persons, and plants. Furthermore, the element plutonium is extremely poisonous. Although relatively harmless when outside of the body, if it is taken into the body by ingestion or through the respiratory track, even a small amount can cause almost immediate death. Plutonium is selectively delivered by the body to the bone marrow, where the alpha emissions can cause significant damage. It has been determined that a dose of 0.6 micrograms of plutonium taken internally is a lethal dose. Thus, plutonium contamination particularly creates a health hazard.
Generally speaking, the scientific community believes that the decay rate of a radioactive nucleus is immutable. However, it is possible to change the decay rate by changing the environment of the emitter. This prior art shows that the decay rate of beta decay and of internal conversion can be changed slightly by varying the chemical composition of an emitter. The present invention is concerned primarily with alpha decay, not investigated by the work of Segre and Wiegand et al, a copy of which was previously made of record. Further the environment change is due to an electrostatic generator. It is not a change in the ambient environment.
According to the accepted theory of beta decay, the decay rate is proportional to .rho.(o)=e.psi.*.psi.(o), the electron charge density at the nucleus. The decay rate may, therefore, be expected to vary with local changes in the electronic environment. It has been found, for instance, that pressure affects the decay rate. Experiments on beta and gamma decay demonstrate that any rearrangement of the electron charge distribution inside the atom may produce a measurable change in decay rate. In all cases investigated, the effect is extremely small. That is, the increase in decay rate is about 0.1%.
The conventional theory of alpha decay is very well known. The decay is described as the tunneling of an alpha particle through the Coulomb potential barrier of the daughter nucleus. The decay constant is determined by the energy of the alpha particle and by the height and width of the barrier. The theory leads to a relationship between decay rate and the change of the daughter nucleus which fits the data extremely well.
The atomic electrons in an alpha emitter also influence the decay rate. In Th.sup.230, for example, these electrons generate a constant potential which extends to the nuclear surface, decreasing the height and width of the Coulomb barrier. Although the corresponding potential energy is relatively small, it has a non-trivial effect on the decay constant. In fact, if all of the atomic electrons were stripped off the thorium atom, the half life would be increased from 80,000 to 146,000 years.
Because of the drawbacks of conventional techniques for reducing the hazards of radioactive waste materials, a need exists to accelerate the decontamination of such materials. The present invention satisfies this need.